1. Field of the Invention
The present invention relates to a hydraulic damper of a damping force adjusting type used with a suspension apparatus for a vehicle such as a motor vehicle.
2. Related Background Art
Among hydraulic dampers mounted in a suspension apparatus for a vehicle such as a motor vehicle, there are hydraulic dampers of a damping force adjusting type capable of suitably adjusting a damping force to improve riding comfort and/or steering stability in dependence upon a road surface condition and a vehicle running condition.
Generally, in such hydraulic dampers of damping force adjusting type, a cylinder containing hydraulic liquid is divided into two cylinder chambers by a piston slidably received within the cylinder and connected to a piston rod, and a main liquid passage and a bypass passage are formed in the piston to communicate the two cylinder chambers with each other. In this case, a damping force generating mechanism comprising an orifice and a disc valve is provided in association with the main liquid passage, and a damping force adjusting valve for adjusting a flow area of the bypass passage is provided in association with the bypass passage. A reservoir is connected to one of the cylinder chambers to compensate for change in the volume of the chambers due to extension and retraction of the piston rod by the compression and expansion of gas.
When the bypass passage is opened through the damping force adjusting valve, flow resistance of the hydraulic liquid flowing between the two cylinder chambers is reduced, thereby decreasing the damping force. Whereas, when the bypass passage is closed, the flow resistance of the hydraulic liquid flowing between the two cylinder chambers is increased, thereby increasing the damping force. In this way, the damping force can be adjusted by opening and closing the damping force adjusting valve.
However, in the hydraulic dampers wherein the damping force is adjusted by adjusting the flow area of the bypass passage, when the piston is moved at a low speed, since the damping force depends upon a feature of the orifice in the liquid passage, the damping force can be changed at a large rate. But, when the piston is moved at an intermediate or high speed, since the damping force depends upon the damping force generating mechanism (disc valve) of the main liquid passage, the damping force cannot be changed at a large rate.
To avoid this, as disclosed in the Japanese Utility Model Laid-open No. 62-155242 (1987), there has been proposed a hydraulic damper wherein a pressure chamber is formed behind a disc valve constituting a damping force generating mechanism for a main liquid passage, and the pressure chamber is communicated with a cylinder chamber at an upstream side of the disc valve through a fixed orifice and is communicated with a cylinder chamber at a downstream side of the disc valve through a variable orifice.
In accordance with such a hydraulic damper of the damping force adjusting type, by opening and closing the variable orifice, a flow area between the two cylinder chambers can be adjusted and pressure in the pressure chamber can be changed to vary the initial pressure for opening the disc valve. In this way, the orifice feature and valve feature can be adjusted, thereby widening an adjusting range for the damping force feature.
However, in the above-mentioned conventional hydraulic dampers of the damping force adjusting type wherein the damping force generating mechanism and the damping force adjusting valve are provided in association with the piston, there arose the following problems. That is to say, if the flow resistance of the piston at a base valve disposed between the cylinder chamber and the reservoir is increased, since an amount of hydraulic liquid flowing into the reservoir is excessively increased during a compression stroke to generate negative pressure in one of the cylinder chambers, the stable damping force cannot be obtained. In this way, since the feature of the damping force of compression side depends upon the flow resistance of the base valve, an adjusting range for the damping force feature of compression side becomes narrower.
Further, since the size of the piston is increased, the stroke of the piston rod is decreased accordingly. In addition, since the damping force adjusting valve is normally actuated through an operation rod inserted in the piston rod, the freedom of design for an attachment portion of the piston rod to a vehicle body is limited.
In the hydraulic dampers of the damping force adjusting type, wherein the pressure chamber is formed behind the disc valve to adjust the opening degree of the disc valve, if the sealing ability for the pressure chamber is weak, the hydraulic liquid will leak from the pressure chamber to decrease the pressure in the chamber, with the result that the stable damping force cannot be obtained. In particular, in the case where a high damping force is set, when the piston is moved at a low speed, the hydraulic liquid is apt to leak, thereby decreasing the damping force.